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+----- Thread: Headway Calcs- choosing 3 or 4 aspect signalling (/showthread.php?tid=1990)
Headway Calcs- choosing 3 or 4 aspect signalling - arjun_082 - 11-09-2014
Hi All,
I'm planning to sit for module 2 this year.
i'm little bit confused with choosing of Aspect.
Can any one please justify why we need to choose 2 Aspect if N>3.66 and
3 Aspect if 3.66>N>2.66..
very thankful in advance..
Regards
Arjun.
RE: Past Papers - Peter - 11-09-2014
(11-09-2014, 05:12 PM)arjun_082 Wrote: Hi All,
I'm planning to sit for module 2 this year.
i'm little bit confused with choosing of Aspect.
Can any one please justify why we need to choose 2 Aspect if N>3.66 and
3 Aspect if 3.66>N>2.66..
very thankful in advance..
Regards
Arjun.
By asking the question, you have illustrated the dangers of learning and quoting a formula without understanding it and it is a credit to you that you are asking. My suggestion to you would be to work back and draw out some examples of what a layout would look like with signal spacing relative to braking distance for a range of numbers for N and you will see that the oft quoted "magic" numbers relate to the boundaries of the requirements (spacing greater than min braking, but not giving excessive braking). At a given point, 3 aspect will start to give you excessive overbraking so you have to go to 2 aspect. The precise breakpoint will of course depend on the tolerance to overbraking that your railway practice has.
Peter
RE: Past Papers - PJW - 11-09-2014
To be honest, from this question I suspect that you have too much to learn to be able to make a reasonable attempt at module 2 this year- it is only a matter of weeks away and this is pretty fundamental.
A fair part of the module 2 Study Pack explains the braking and headway calculations in significant detail; I have made my best attempt to explain there and so I have to conclude either
a) you have not read, or
b) you have read but not comprehended.
Whichever it is I don't see what I can do to help more here, other than give a very high level description in case you have "been unable to see the wood for the trees".
Outline is
1. The more signals in a given length the more capacity but also the greater the cost.
2. You must provide the capacity specified, but at the most affordable cost so therefore don't incur costs to achieve more than is strictly needed.
3. For continuous 3 aspects signalling, the signals must be at least braking distance apart, but not more than 1.33 times braking distance (to limit the risks associated with drivers getting accustomed to the sections actually being overbraked.)
4. If 3 aspects at minimum spacing cannot give the required capacity, then 4 aspects must be provided.
5. Conversely if 3 aspects at maximum spacing provides far more capacity than is needed, significant money can be saved by using isolated 3 aspect signalling comprising widely spaced 2 aspect Red /Green signals, each with their own individual Yellow / Green distants.
6. However it is stupid to provide isolated 3 aspect signalling unless it is actually noticeably cheaper than widely spaced continuous 3 aspect signalling because you are throwing away capacity but saving nothing.
Forget quoting a formula and some "magic numbers" that you clearly don't understand. Seek to understand the signalling logic which underpins it, draw it out and think.
We are trying to work out how many braking distances "fit" within the than headway distance and 2.66 is twice 1.33 and 3.66 is one more 2.66.
Three block section's worth of 3 aspect signalling uses 3 3-aspect signals; two block section's worth of isolated 3 aspect signalling uses 4 2-aspect signals.
(11-09-2014, 05:12 PM)arjun_082 Wrote: Hi All,
I'm planning to sit for module 2 this year.
i'm little bit confused with choosing of Aspect.
Can any one please justify why we need to choose 2 Aspect if N>3.66 and
3 Aspect if 3.66>N>2.66..
very thankful in advance..
Regards
Arjun.